1. Field
Embodiments disclosed herein relate generally to cutting elements for drilling earth formations. More specifically, embodiments disclosed herein relate to the interface design in shear cutters.
2. Background Art
Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and cutter pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation.
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.
PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
In PDC bits, polycrystalline diamond compact (PDC) cutters are received within cutter pockets, which are formed within blades extending from a bit body, and are typically bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
In a typical PDC cutter, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide, to form a cutting structure. PCD comprises a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
PCD may be formed by subjecting a volume of diamond grains to certain high-pressure/high-temperature (“HPHT”) conditions in the presence of a sintering aid or binder. Conventionally, the sintering aid or binder is provided in the form of a solvent metal catalyst material, such as one or more elements from Group VIII of the Periodic table. The solvent metal catalyst may be added and mixed with the diamond grains prior to HPHT processing and/or may be provided during the HPHT process by infiltration from a substrate comprising the solvent metal catalyst as one of its constituent materials.
A conventional PDC cutter may be formed by placing a cemented carbide substrate into a HPHT container. A diamond layer may be formed upon the substrate by placing a mixture of diamond grains or diamond grains and catalyst binder atop the substrate in the container. The container is then loaded into a HPHT device that is configured and operated to subject the container and its contents to a desired HPHT condition. In doing so, metal binder migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide. The deposited diamond body is often referred to as a “diamond layer”, a “diamond table”, or an “abrasive layer.”
An example of a prior art PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIG. 1. The drill bit 100 includes a bit body 110 having a threaded upper pin end 160 and a cutting end 140. The cutting end 140 typically includes a plurality of ribs or blades 120 arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body 110. Cutting elements, or cutters, 180 are embedded in the blades 120 at predetermined angular orientations and radial locations relative to a working surface 190 and with a desired back rake angle and side rake angle against a formation to be drilled.
A plurality of orifices 130 are positioned on the bit body 110 in the areas between the blades 120, which may be referred to as “gaps” or “fluid courses.” The orifices 130 are commonly adapted to accept nozzles. The orifices 130 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 120 for lubricating and cooling the drill bit 100, the blades 120 and the cutters 180. The drilling fluid also cleans and removes the cuttings as the drill bit 100 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 180 may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 100 toward the surface of a wellbore (not shown).
Conventional PCD includes 85-95% by volume diamond and a balance of the binder material, which is present in PCD within the interstices existing between the bonded diamond grains. Binder materials that are typically used in forming PCD include Group VIII elements, with cobalt (Co) being the most common binder material used.
Conventional PCD is stable at temperatures of up to 700-750° C., after which observed increases in temperature may result in permanent damage to and structural failure of PCD. In particular, heat caused by friction between the PCD and the work material causes thermal damage to the PCD in the form of cracks, which lead to spalling of the diamond layer and delamination between the diamond layer and substrate. This deterioration in PCD is due to the significant difference in the coefficient of thermal expansion of the binder material, which is typically cobalt, as compared to diamond. Upon heating of PCD, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the PCD. High operating temperatures may also lead to back conversion of the diamond to graphite causing loss of microstructural integrity, strength loss, and rapid abrasive wear.
In order to overcome this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure (either a thin volume or the entire body) to at least reduce the damage experienced from different expansion rates within a diamond-cobalt composite during heating and cooling. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, typically nitric acid or combinations of several strong acids (such as nitric and hydrofluoric acid) may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. By leaching out the cobalt, thermally stable polycrystalline (“TSP”) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability with less effect on impact resistance. As used herein, the term thermally stable polycrystalline (TSP) includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by reinfiltrating the volume with a secondary material. An example of reinfiltration can be found in U.S. Pat. No. 5,127,923.
However, some of the problems described above that plague PCD cutting elements, i.e., chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table, are also often encountered in TSP cutters or other types of cutters having an ultra hard diamond-like cutting table such as polycrystalline cubic boron nitride (PCBN) bonded on a cemented carbide substrate. It has been observed that carbide substrates may have a higher coefficient of thermal expansion than a diamond layer (e.g., PCD, TSP). TSP materials, in particular, have a coefficient of thermal expansion that is sufficiently different from that of conventional substrate materials (such as WC-Co). Thus, during sintering, for example, both the cemented carbide body and diamond layer are heated to elevated temperatures forming a bond between the diamond layer and the cemented carbide substrate. As the diamond layer and substrate cool down, the substrate shrinks more than the diamond because of the carbide's higher coefficient of thermal expansion. Consequently, stresses referred to as thermally induced stresses, or residual stresses, are formed at the interface between the diamond and the substrate. Further, different contractions between the diamond layer and carbide substrate generate stresses in both bodies.
Furthermore, in some prior art disclosures, the difference in the coefficient of thermal expansion between conventional substrate materials and TSP has resulted in mounting TSP bodies directly to a device for use rather than using an adjoining substrate. In particular, the difference in thermal expansion between the TSP body and a substrate, and the poor wetability of the TSP body diamond surface due to the substantial absence of solvent metal catalyst, makes it very difficult to bond TSP to conventionally used substrates.
Moreover, residual stresses are formed on the diamond layer from a mismatch in the bulk modulus between the diamond layer and substrate. Specifically, the high pressure applied during the sintering process causes the carbide to compress more than the diamond layer due to the carbide's lower bulk modulus. After the diamond is sintered onto the carbide and the pressure is removed, the carbide tries to expand more than the diamond imposing a tensile residual stress on the diamond layer. These stresses may induce larger stresses, which may ultimately lead to material failure, because diamond and substrate materials typically have a high modulus (i.e., stiffness).
The cooling down effect (caused by different coefficients of thermal expansion) and the pressure release effect (caused by different bulk modulus) counteract with each other. The cooling down effect over powers the pressure release effect under commonly used sintering conditions, thereby leaving different net contractions in the diamond layer and carbide substrate.
To avoid these issues, some prior art designs use non-planar interfaces (“NPI”) between the substrate and a diamond cutting layer. However, the formation of a NPI becomes more difficult to achieve when sintering a preformed diamond layer to a carbide substrate because any imprecision between mating non-planar surfaces of the diamond and substrate may cause cracking in the diamond layer. Some prior art embodiments try to improve mating precision between preformed diamond layers and substrates by using substrate material in powder form, such as carbide powder, between the non-planar surfaces.
FIGS. 2A and 2B show exemplary prior art cutting elements. The cutting element 200 shown in FIG. 2A has a conventional planar interface 202 formed between a diamond table 220 and a carbide substrate 270. The cutting element 200 shown in FIG. 2B has a conventional NPI 202 formed between a diamond table 220 and a substrate 270. A portion of the substrate 270 extends into the diamond table 220. In an exemplary prior art embodiment, the substrate 270 may be preformed into a carbide body, and diamond powder and optionally catalyst material may be placed on the upper surface of the substrate 270. Upon HPHT processing of the cutting element 200, diamond to diamond bonding occurs to form the diamond table 220. Although prior art NPIs, such as the one shown in FIG. 2B, may have reduced amounts of residual stress when compared to cutting elements having planar interfaces, such as the one shown in FIG. 2A, prior art configurations of diamond cutting elements continue to exhibit failure from residual stresses.
Accordingly, there exists a continuing need for developments in improving the life of cutting elements.